1. Field
The present disclosure generally relates to a system and method for evaluating bonds between structures, and, more particularly, to a system and methods for reducing tensile stresses generated during non-destructive evaluation of bonded structures.
2. Background
Bonds are widely used in a variety of structural applications, and more specifically, primary composite structures are often bonded together in select aerospace applications. Generally, the strength of the bond between composite structures must be known and certifiable if the bond is used in a primary structure application. Conventional methods for measuring bond strength generally involve static and dynamic proof testing of entire structural assemblies, subjected to simulated loads and are monitored for strain levels, which are then correlated to strength values. The structure that is tested, however, is generally a test assembly and is not used in the final assembly of the aircraft. Further, smaller component testing of individual bonds is conducted, but the components are also test assemblies and are not a part of the operational vehicle structure. Moreover, the smaller components are most frequently destructively tested.
Non-destructive inspection (NDI) methods also exist for composite structures assembled with adhesive bonds. Among other techniques, laser bond inspection (LBI) has proven useful. Laser bond inspection is a method of testing the strength of bonds between composite structures using stress waves. In this technique, weak bonds may be “pulled apart” by tension waves traveling through the structure.
With LBI, as shown in FIGS. 1(A)-1(D), a laser beam 102 is generally directed at the front surface of a bonded structure 100, which generates mechanical waves in the form of compression waves 104 that travel through the structure towards a back surface of the bonded structure 100. When the compression wave 104 reaches the back surface of the bonded structure 100, the compression wave 104 is reflected back from that surface, producing a tension wave 112 (FIG. 1c), that propagates back towards the front surface of the structure 100. The tension wave 112 applies tension to the internal makeup of the bonded structure 100, including any bond 106 holding the components of the structure 100 together.
If a tension wave 112 of sufficient strength encounters a weak bond, the bond will fail, for example, by separating. A failed bond may be detected by various techniques and methods, including, for example, ultrasonics, x-rays, and acoustics, among others commonly known in the art.
As better shown in FIG. 1(B), generally, when a compression wave 104 penetrates the structure 100 and reaches the bond 106, a portion of the compression wave 104 transfers through the bond 106 to form a transmitted wave 110 that is amplified due to the differences in the material properties, namely the wave speed (i.e., the speed of sound in the material) and density, of the bonded materials. Another portion of the compression wave 104 reflects off of the bond 106, producing a reflected compression wave 108 that propagates back towards the front surface of the composite structure 100.
As shown in FIG. 1(C), transmitted wave 110 reflects off of the back surface of the composite structure 100 producing a first tension wave 112 that propagates back towards the bond line 106. Compression wave 108 reflects off of the front surface of the composite structure 100 producing a second tension wave 114 that also propagates back towards the bond 106. Typically, the tension wave 112 reaches the bond 106 first and upon encountering the bond, this tension wave 112 subjects the bond 106 to a desired tensile stress and continues to propagate toward the front surface of the composite structure 100.
The problem with current LBI methods, as shown in FIG. 1(D), is that after tension wave 112 reaches the bond 106, it is generally transmitted through the bond 106 to combine with tension wave 114, shown in FIG. 1(C), to produce an unwanted tensile stress spike 116 in a region of the composite structure 100 between the front surface and the bond 106. This stress spike 116 often results in cracking or other mechanical failures in the composite structure 100. Thus, if the bond 106 is as strong as the composite material, the laser exposure may cause the composite material to fracture before the bond 106 is broken.
Accordingly, a need therefore exists for a system and methods for reducing tensile stress generated during non-destructive inspection of bonded composite structures.